1. Technical Field
The present invention relates to submarine fiber optics and, in particular, to modulation techniques for effective submarine transmission.
2. Description of the Related Art
As global interne bandwidth demand continues to grow, fiber optic submarine systems will need spectrally efficient modulation formats with high sensitivity. While coherent optical systems that recover the optical electric field and enable digital compensation of transmission impairments achieve the highest spectral efficiencies, these systems have high complexity and high cost. Moreover, existing submarine links utilize inline dispersion management, which incur high nonlinearity penalties. It has been demonstrated that, at a spectral efficiency of one bit per Hz or less, coherent systems may not necessarily provide any benefit over non-coherent detection schemes such as differential detection, as carrier recovery in coherent systems is sensitive to nonlinear phase noise.
Previous optical systems have used modulation formats with four signal points (two uncoded bits) per symbol such as single-polarization differential quaternary phase-shift keying (SPDQPSK) or dual-polarization differential binary phase-shift keying (DP-DBPSK). Systems based on SP-DQPSK have generally yielded suboptimal performance in dispersion-managed systems, where fiber nonlinearity leads to greater phase distortion than either amplitude or polarization distortion.
Although DP-DBPSK has good robustness against phase distortion, it requires the demultiplexing of two polarizations at the receiver. While polarization tracking can be used, it leads to increased system complexity and potential vulnerability to rapid polarization changes over long transmission distances. It is also possible to encode information in the two polarization tributaries in such a manner that the tributaries are orthogonal to a standard DQPSK demodulator. One example is to transmit DBPSK in each fiber polarization, but offset their respective center frequencies by one quarter of the baud rate. However, this scheme broadens the signal spectrum, making the system more vulnerable to narrow optical filtering.
Dual-polarization transmission formats also suffer another problem: in systems with periodic amplification impacted by amplified spontaneous emission (ASE) noise, differential detection results in each signal polarization contributing a signal-ASE beat term, leading to a 3 dB power penalty if symbols in the two polarization tributaries are aligned in time. In practice, this penalty can be negated by filtering the unwanted signal polarization before differential detection, but this requires polarization tracking. Alternatively, one can use sufficiently narrow pulses much less than a 50% duty cycle and then offset the symbols in the two polarizations in time so that their signal-ASE beat terms do not overlap. However, this doubles the signal bandwidth.
As such, it is clear that none of the existing methods are adequate for the increasing needs of submarine transmission.